Hi,Can someone tell me please if I am right here. If you are looking to have a unique key, and you take two columns to make a unique key, but one of th ecolumns have nulls in it; then it is not a unique key.Example:ID number, Reference number Null , 1256 10456 , Null

The set of columns in a UNIQUE constraint isn't necessarily a key. Keys by definition don't permit nulls and so a column that permits nulls can't be part of any key.

SQL Server's UNIQUE constraint behaves differently from UNIQUE constraints in standard SQL. In ISO Standard SQL, a UNIQUE constraint actually permits duplicate rows if any of its columns includes a null. SQL Server UNIQUE constraints do not permit duplicate rows - nulls are instead treated as equal values for the purpose of evaluating whether the constraint is violated or not.

I highly recommend you avoid nulls in UNIQUE constraints. You can always redesign it without the nulls by creating the constraint on a new table and then only populating that table with the non-nullable values.